Flexography is a method of printing that is commonly used for high-volume printing runs. It is usually employed for printing on a variety of soft, coarse, or easily deformed materials including but not limited to, paper, paperboard stock, corrugated board, polymeric films, fabrics, metal foils, and laminates.
Flexographic printing members are sometimes known as “relief” printing members and are provided with raised relief images onto which ink is applied for application to a printable material and the relief “floor” should remain free of ink. The flexographic printing precursors are generally supplied with one or more imagable (or engravable) layers that can be disposed over a backing layer or substrate. Flexographic printing also can be carried out using a flexographic printing cylinder or seamless sleeve having the desired relief image. These flexographic printing members can be provided from flexographic printing precursors that can be imaged through a photomask or laser-ablatable mask (LAM) over a photosensitive layer, or they can be imaged by direct laser engraving of a laser-engravable layer that is not necessarily photosensitive.
Flexographic printing precursors having laser-ablatable mask layers over photosensitive layers are described for example in U.S. Pat. No. 5,719,009 (Fan). A developer is used to remove non-polymerized material from the photosensitive layer and the non-ablated portions of the mask layer.
There has been a desire in the industry for a way to prepare flexographic printing members without the use of UV-cured photosensitive layers that require liquid processing to remove non-imaged composition and mask layers. Direct laser engraving of precursors to produce relief printing plates and stamps is known, but the need for relief image depths greater than 500 μm creates a considerable challenge when imaging speed is also an important commercial requirement. In contrast to laser ablation of mask layers that require low to moderate energy lasers and fluence, direct engraving of a relief-forming layer requires much higher energy and fluence. A laser-engravable layer must also exhibit appropriate physical and chemical properties to achieve “clean” and rapid laser engraving (high sensitivity) so that the resulting printed images have excellent resolution and durability.
Flexographic printing plate precursors used for infrared radiation (IR) laser-engraving can comprise an elastomeric or polymeric composition that includes one or more infrared radiation absorbing compounds. When the term “imaging” is used in this application in connection with “laser engraving”, it refers to ablation of the background areas while leaving intact the areas of the element that will be inked and printed in a flexographic printing station or press.
Commercial laser engraving has been typically carried out using carbon dioxide lasers. While they are generally slow and expensive to use and have poor beam resolution, they are used because of the advantages of direct thermal engraving. Infrared (IR) fiber lasers can also be used because these lasers provide better beam resolution, but are very expensive. Direct laser engraving is described, for example, in U.S. Pat. Nos. 5,798,202 and 5,804,353 (both Cushner et al.) in which various means are used to reinforce the elastomeric layers.
Flexographic printing plate precursors for near-IR laser-engraving generally comprise an elastomeric or polymeric system that is thermoset by a polymerization reaction and includes inorganic fillers and infrared absorbing compounds. During recent years, infrared laser diodes are becoming increasingly inexpensive and more powerful and consequently are becoming more useful for laser-engraving of thick layers such as are found in flexographic printing precursors. Such lasers require the presence of radiation absorbing dyes or pigments in the flexographic printing precursors as they generally operate around wavelengths of 800 nm to 1200 nm. They have the potential to enable faster engraving, higher print quality, and more reliable engraving than obtained with carbon dioxide lasers. It is advantageous to optimize engraving speed by formulating printing plates with higher sensitivity to give higher productivity in printing plate production. Engraving systems can be made by using arrays of laser diodes as throughput also depends on the number of laser diodes being used but there is a balance between the cost of engraving heads that depends on the number of diodes and their combined output power. Laser engraving using infrared diodes instead of carbon dioxide provides an opportunity for higher quality because the wavelength of the diode radiation at 800-1200 nm is so much smaller than that of carbon dioxide at 10.7 μm.
In the approach to formulation of laser-engravable flexographic printing precursors by crosslinking to form thermoset materials, ablation of thermoplastic materials results in melted portions around the ablated areas and sometimes re-deposition of ablated material onto the ablated areas. This is because it is inevitable that during engraving there is heat flowing to non-engraved areas that is insufficient for ablation but sufficient for melting, as described in U.S. Patent Application Publication 2004/0231540 (Hiller et al.).
A number of elastomeric systems have been described for construction of laser-engravable flexographic printing precursors including a mixture of epoxidized natural rubber and natural rubber in a laser-engravable composition. Engraving of a rubber is also described by S. E. Nielsen in Polymer Testing 3 (1983) pp. 303-310. U.S. Pat. No. 4,934,267 (Hashimito) describes the use of a natural or synthetic rubber, or mixtures of both, such as acrylonitrile-butadiene, styrene-butadiene and chloroprene rubbers, on a textile support. “Laser Engraving of Rubbers—The Influence of Fillers” by W. Kern et al., October 1997, pp. 710-715 (Rohstoffe Und Anwendendunghen) describes the use of natural rubber, nitrile rubber (NBR), ethylene-propylene-diene terpolymer (EPDM), and styrene-butadiene copolymer (SBR) for laser engraving.
EP 1,228,864A1 (Houstra) describes liquid photopolymer mixtures that are designed for UV imaging and curing, and the resulting flexographic printing plate precursors are laser-engraved using carbon dioxide lasers operating at about 10 μm wavelength. Such printing plate precursors are unsuitable for engraving using more desirable near-IR absorbing laser diode systems. U.S. Pat. No. 5,798,202 (noted above) describes the use of reinforced block copolymers incorporating carbon black in a layer that is UV cured and remains thermoplastic. Such block copolymers are used in many commercial UV-sensitive flexographic printing plate precursors. As pointed out in U.S. Pat. No. 6,935,236 (Hiller et al.), such curing would be defective due to the high absorption of UV as it traverses through the thick imagable layer. Although many polymers are suggested for this use in the literature, only extremely flexible elastomers have been used commercially because flexographic layers that are many millimeters thick must be designed to be bent around a printing cylinder and secured with temporary bonding tape and both must be removable after printing.
U.S. Pat. No. 6,776,095 (Telser et al.) describes elastomers including an EPDM elastomeric rubber and U.S. Pat. No. 6,913,869 (Leinenbach et al.) describes the use of an EPDM elastomeric rubber for the production of flexographic printing plates having a flexible metal support. U.S. Pat. No. 7,223,524 (Hiller et al.) describes the use of a natural rubber with highly conductive carbon blacks. U.S. Pat. No. 7,290,487 (Hiller et al.) lists suitable hydrophobic elastomers with inert plasticizers. U.S. Patent Application Publication 2002/0018958 (Nishioki et al.) describes a peelable layer and the use of rubbers such as EPDM and NBR together with inert plasticizers such as mineral oils.
EPDM elastomeric rubbers were commercially developed in the 1960's and provide certain advantages for use in flexographic printing plate precursors. Unlike SBR (styrene-butadiene rubber), which was developed as an inexpensive replacement for natural rubber in tires, EPDM elastomeric rubbers provide higher performance, making them more useful for non-tire uses. EPDM elastomeric rubbers have a fully saturated molecular backbone that provides excellent ozone resistance, weatherability, and flexibility at low temperatures.
In EPDM elastomeric rubbers, the compression set and aging depend largely on the crosslinking agent (vulcanizing agent) used in formulating a composition. Carbon-carbon bonds that are provided by peroxide vulcanizing agents are more expensive to provide than carbon-carbon bonds provided by sulfur vulcanizing agents. Polysulfide vulcanizing compositions provide higher strength while monosulfide links provide better aging properties and stability. However, EPDM elastomeric rubber vulcanization using sulfur vulcanizing agents tends to be less efficient than peroxide vulcanization.
An increased need for higher quality flexographic printing precursors for laser engraving has highlighted the need to solve performance problems that were of less importance when quality demands were less stringent. However, it has been especially difficult to simultaneously improve the flexographic printing precursor in various properties because a change that can solve one problem can cause or worsen another problem.
For example, the rate of engraving is an important consideration in laser engraving of flexographic printing precursors. Throughput (rate of imaging multiple precursors) depends upon printing plate precursor width because each precursor is engraved point by point. Engraving, multi-step processing, and drying of UV-sensitive precursors is time consuming but this process is independent of printing plate size, and for the production of multiple flexographic printing plates, it can be relatively fast because many flexographic printing plates can be passed through the multiple stages at the same time.
In contrast, throughput using laser-engraving is somewhat determined by the equipment that is used, but if this is the means for improving engraving speed, the cost becomes the main concern. Improved engraving speed is thus related to equipment cost. There is a limit to what the market will bear in equipment cost in order to have faster engraving. Therefore, much work has been done to try to improve the sensitivity of the flexographic printing plate precursors by various means.
U.S. Patent Application Publication 2009/0214983 (Figov et al.) describes the use of additives that thermally degrade during engraving to produce gaseous products. U.S. Patent Application Publication 2008/0194762 (Sugasaki) suggests that good engraving sensitivity can be achieved using a polymer with a nitrogen atom-containing hetero ring. U.S. Patent Application Publication 2008/0258344 (Regan et al.) describes laser-ablatable flexographic printing precursors that can be degraded to simple molecules that are easily removed.
Copending and commonly assigned U.S. Ser. No. 12/748,475 (filed Mar. 29, 2010 by Melamed, Gal, and Dahan) describes flexographic printing precursors having laser-engravable layers that include mixtures of high and low molecular weight EPDM rubbers, which mixtures provide improvements in performance and manufacturability.
As flexographic engraving (sensitivity) is improved, the need for print quality and consistency increases. In addition, there is a need to make manufacturing as consistent as possible. Laser-engravable compositions to be compounded tend to have relatively high viscosity, presenting challenges in ensuring excellent mixing of the essential components. This problem is addressed with the invention described in U.S. Ser. No. 12/748,475 noted above by incorporating a low viscosity EPDM rubber into the composition. Compression recovery can then be a challenge because a good compression rate and printability are generally associated with high molecular weight elastomers in relatively high viscosity compositions.
Copending and commonly assigned U.S. Ser. No. 12/173,430 (filed Jun. 30, 2011 by Melamed, Gal, and Dahan) describe the use of laser-engravable compositions comprising CLCB EPDM elastomeric rubbers and vulcanizing compositions that can include mixtures of peroxides or sulfur-containing compounds. The EPDM elastomeric rubbers used in these compositions generally comprise less than 8 weight % of polyene recurring units.
The cost of manufacturing flexographic printing precursors is an important consideration during development. Another important consideration is engraving throughput, which is dependent upon the speed of curing and the speed of engraving. As noted above, peroxides provide faster curing than sulfur vulcanizing agents, but the use of these sulfur vulcanizing agents provides other advantages such as faster engraving speed. There is a need to increase curing speed with the use of sulfur vulcanizing agents without the loss of other desirable properties such as increased engraving throughput.